What is the binary representation of a valid subnet mask?

The binary representation of a valid subnet mask must be an unbroken sequence of binary ones followed by binary zeros. This convention ensures that the subnet mask can clearly define the network portion and the host portion of an IP address. In this structure, the binary ones indicate the bits that represent the network, while the subsequent zeros represent the bits available for host addresses.

For example, subnet masks such as 11111111.11111111.11111111.00000000 (or 255.255.255.0 in decimal) demonstrate this principle perfectly, with all ones coming first consecutively before any zeros. This structure is crucial for routing and IP address management, as it allows devices on a network to determine which part of an IP address is used for the network and which part is designated for hosts within that network.

Other variations, such as a broken string of ones and zeros, do not conform to the standard format of subnet masks and therefore cannot be valid. Likewise, options that introduce incorrect digits, like twos, or suggest random arrangements of binary digits would render the subnet mask ineffective for its intended purpose. Hence, the only acceptable format is the consecutive ones followed by zeros.